1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying apparatus or a printer, and more particularly to gamma (γ) conversion for use therein
2. Related Background Art
For adjusting image density in an image forming apparatus, there are known digital image processing methods and analog adjustment methods. The digital image processing methods include an adjustment with luminance-density conversion means for converting a received original luminance signal into an original density signal, an adjustment with output density conversion means for converting an original density signal into an output density signal, and an adjustment constituted by an inverse function of the printer characteristics representing an output density signal corresponding to an output signal. In general, the adjustment by the gamma conversion means is easiest for adjusting the image density, and is more direct and accurate in determining the printer characteristics.
For analog adjustment, it is common to adjust the potential of the latent image or the development potential. There are other adjustable parameters such as toner amount of the development means, development frequency, various adjusting gaps in the development means and a gap between the development means and the electrophotographic photosensitive member, but the latent image potential and the development potential has conventionally been employed in consideration of ease of the adjustment. Particularly, the most common parameter is the development contrast which is the difference between the latent image potential inducing the toner adhesion on the electrophotographic photosensitive member and the development potential of the development means.
The digital image adjustment represented by the adjustment of the content of conversion of the gamma conversion means is capable of adjusting the density in the halftone area relatively according to the desire, but cannot adjust the density of an area in which the toner is deposited with the maximum amount (hereinafter represented as Dmax) or the fog level in an area where the toner is not deposited.
On the other hand, the analog image adjustment. represented by the adjustment of the development contrast can adjust not only the density of the halftone area but also Dmax in mutual linkage. The fog level in the area without the toner adhesion scarcely changes unless the back contrast, relating to the difference between the development potential and the potential in the area without the toner adhesion, is altered.
Also, the digital image adjustment and the analog image adjustment as described in the foregoing may conventionally be designated individually, but there has not existed an algorithm or a sequence capable of automatically linking the two in the image forming apparatus.
It is to be noted that there may result a situation where the development contrast has to be altered not fur the purpose of the image adjustment, for example for reducing the toner consumption amount or for avoiding the leakage of the charging means for providing the photosensitive member with a potential. As explained in the foregoing, a change in the development contrast alone results in a change in the density of the halftone area and Dmax in linkage. The resulting image therefore provides an impression of significantly deteriorated image quality.